Carbon monoxide will bind with iron in the hemoglobin of the blood and cause the blood to become unable to carry oxygen to the cells in the body. This happened to my dad when I was a small child and his company had a gasoline driven welding machine running in what they thought was a vented building. He turned cherry red and luckily one of the other guys noticed him getting stumbling and slurring words and got him to the fresh air and a hospital. This has remained with me for all these many years.
That said, the CO detector is a VERY important item in closed environment like an RV.